The present invention relates in general to optical systems for shaping illumination from a diode-laser. The invention relates in particular to an optical system for projecting light from a diode-laser into a narrow line.
Diode-lasers are commonly used as sources of illumination in various graphics applications such as display systems, optical printing systems and optical recording systems. For such applications many optical systems have been devised to optically shape the characteristic astigmatic light output of a diode-laser into a symmetrical anastigmatic form that can be focussed into an biaxially symmetrical spot or illuminating area. By way of example, a very small, uniform spot of light projected from a diode-laser can be used to record correspondingly small spots on a light sensitive medium. A plurality of small spots recorded over an area can be used to record a graphic image or pattern. A line of small spots in a particular sequence and spacing can be used to optically record data.
There are other applications for diode-laser illumination which would be possible if the diode-laser light could be projected into the form of a fine line having a width no greater than about 6.0 micrometers (xcexcm). Such applications include laser microwelding and laser machine alignment.
In one aspect, the present invention is directed to providing an illumination apparatus for projecting the light-output of a semiconductor-laser into a fine line. The line preferably has a width less than about 6.0 xcexcm. One preferred embodiment of the inventive apparatus includes a semiconductor-laser providing light output in first and second mutually perpendicular axes. The light-output propagates in a direction mutually perpendicular to the first and second axes. The inventive apparatus includes an optical system arranged cooperative with the semiconductor-laser to focus the light-output in the first and second axes at respectively first and second focal points spaced apart in the propagation direction. At a plane intersecting the first focal point perpendicular to the direction of propagation, the focused semiconductor-laser light-output is formed into a line of light having a width in the first axis and a length in the second axis.
Preferably the first focal point is closer to the optical system than the second focal point. The diode laser may be a semiconductor laser emitting light-output having a different divergence in characteristic fast and slow axes. The fast and slow axes correspond to above discussed first and second axes respectively. The semiconductor laser may be a vertical-cavity surface-emitting laser (VCSEL). Such a laser has an essentially symmetrical light-output, in which case the above-discussed first and second axes correspond to respectively Y and X axes of the optical system.
In one preferred embodiment, the optical system includes first, second and third lenses spaced apart in consecutive numerical order in the direction of propagation. The first and third lenses each have positive dioptric power in both the first and second axes. The second lens has zero dioptric power in the first axis and positive dioptric power in the second axis. The first lens is spaced apart from the semiconductor laser by a distance equal to about its focal length. The second lens is spaced apart from the third lens by a distance greater than the second-axis focal length. The third lens has a focal length greater than the focal length of the first lens, and the second lens has a second-axis focal length greater than the focal length of the third lens. The first focal point is closer to the optical system than the second focal point
In one example of the inventive line-illumination system incorporating the above-exemplified optical system the semiconductor is a single-mode edge-emitting diode laser. A line of light having a width of about 3.0 xcexcm is projected at a distance of about 0.23 inches from the optical system. The projected line of light has a length of about 170.0 xcexcm. The first and second back-focal lengths of the optical system are about 0.23 and 0.80 inches respectively. The 3.0 xcexcm line-width and all other line-widths referred to herein are the widths measured at across the 1/e2 points of the lines.